1. Field of the Invention
The present invention relates to a process for the extrusion of liquid crystal polymers to produce shaped articles. More particularly, the present invention relates to an advantageous process for extruding liquid crystal polymers to produce articles having high mechanical properties and cross-sectional dimensions which can closely approximate those of the extrusion die.
2. Description of the Prior Art
In conventional profile extrusion, the dies generally comprise a converging entry region and a land region. While one of the most important aspects of profile extrusion is to control the extrudate profile, it is extremely difficult to control the product profile when extruding conventional flexible polymers since their extrudates do not conform to the die shape. Such incapability of flexible polymers to melt extrude a profile which will accurately correspond to the die shape is due to the elasticity of the polymers and their recoiling upon exiting the die, thereby resulting in extrudate swell. This distortion of extrudate size and shape can generally be reduced by using increased land length and by distorting the profile of the die. For example, in order to extrude a square profile, the die employed in the extrusion of a flexible polymer is generally of a concave configuration around the center. A strictly square die would yield a pillow shaped extrudate.
Thermotropic liquid crystal polymers are known to generally not exhibit elastic recoil when extruded due to their molecular conformation. Thus, melt extruded profiles of liquid crystalline polymers generally tend to conform very closely to die profiles. When forming a melt extruded profile from a liquid crystal polymer, therefore, an extrusion die can be generally selected having a shape which corresponds to the cross-sectional configuration of the profile to be formed, with the exception, however, that the die orifice dimensions are larger than the dimensions of the resulting profile due to a drawdown of the extrudate. Liquid crystal polymer extrudates are generally drawn while in the melt phase immediately adjacent to the extrusion orifice and prior to complete solidification in order to advantageously induce relatively high molecular orientation coextensive with the length of the profile. Such orientation of the polymer molecules contributes to the enhanced mechanical properties of the final product. The extent of such drawdown is influenced by the haul-off speed under which the profile is collected. The resulting drawdown ratio is defined as the ratio of the die cross-sectional area to that of the cross-sectional area of the fully solidified extrudate, which draw ratios (can range between 4 and 100 or more and) are commonly at least 10.
In many instances, however, e.g., in various industrial applications, the dimensions of a product need to be exacting. When such a product is to be extruded, the extrusion process, to be efficient and effective, would thereby need to consistently provide products of the same dimensions. To consistently achieve this result can be very tricky when one has to rely on a distortion of the extruded profile via extrudate swell in order to obtain the properly sized product. Large drawdown of the extruded profile, as in the case of liquid crystal polymers, can also lead to some inconsistencies in the size of the articles produced. In general, any large distortion in the profile dimensions vis-a-vis the extrusion die allows room for inconsistencies. Thus, a process which would consistently yield a product of reproducible dimensions would be of benefit to the art. It is also important, however, that any such extrusion process for attaining the desired consistency and reproducibility in product dimensions does not sacrifice the mechanical properties of the resulting product.
Accordingly, it is an object of the present invention to provide a process for extrusion of shaped articles whereby the dimensions of the extruded article can be consistently reproduced.
It is another object of the present invention to provide a process for the extrusion of shaped articles whereby the articles exhibit high mechanical properties.
Yet another object of the present invention is to provide a process for extruding shaped articles of good mechanical properties whereby the dimensions of the extrudate closely approximate the dimensions of the die orifice.
Still another object of the present invention is to provide a process for extruding liquid crystal polymers to produce articles of good mechanical properties while utilizing minimal drawdown.
Still another object of the present invention is to provide a novel process for extruding liquid crystal polymers of a high viscosity, e.g., a high molecular weight.
Yet another object of the present invention is to provide a process for extruding liquid crystal polymers at higher rates (for a given pressure drop and viscosity) and with little distortion of the profile while attaining good mechanical properties.
Still another object is to provide a process for extruding liquid crystal polymers at higher rates and at lower temperatures (for a given extrusion rate and pressure drop) to thereby result in minimal degradation of the polymers.
These and other objects, as well as the scope, nature and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description and appended claims.